In the field of semiconductor manufacturing, optical lithography has been the mainstream approach used in patterning semiconductor devices. In typical prior art photolithography processes, UV light is projected onto a silicon wafer coated with a thin layer of photosensitive resist (photoresist) through a mask that defines a particular circuitry pattern. Exposure to UV light, followed by subsequent baking, induces a photochemical reaction which changes the solubility of the exposed regions of the photoresist. Thereafter, an appropriate developer, usually an aqueous base solution, is used to selectively remove the portion of the photoresist affected by the photochemical reaction, either in the exposed regions (positive-tone photoresists), or in the unexposed regions (negative-tone photoresists). The pattern thus defined is then imprinted on the silicon wafer by etching away the regions that are not protected by the photoresist with a dry or wet etch process.
One type of photoresist employed in the prior art is a chemically amplified photoresist (CAR) which uses acid catalysis (H. Ito, Adv. Polym. Sci. 2005, 172, 37; G. Wallraff, W. Hinsberg, Chem. Rev. 1999, 99, 1801). A typical prior art chemically amplified photoresist, for example, is formulated by dissolving an acid sensitive polymer and a photoacid generator (PAG) in a casting solution (J. Crivello, J. Polym. Sci. Part A: Polym. Chem. 1999, 37, 4241; H. Ito, C. G. Wilson, Polym. Eng. Sci. 23, 1012 (1983)). A chemically amplified photoresist is especially useful when relatively short wavelength radiation is employed, including deep UV radiation (DUV) 150-350 nm wavelengths, and mid-UV radiation (MUV), e.g., 350-450 nm wavelengths. The shorter wavelengths are typically desired to increase resolution, and thus, decrease feature size of the semiconductor devices, but fewer photons are radiated for a given energy dose.
Accordingly, higher exposure doses are typically required when using UV radiation to obtain a sufficient photochemical response in the photoresist unless a chemically amplified photoresist is employed. In a chemically amplified photoresist, acid sensitivity of the base polymer exists because acid sensitive side chain groups are bonded to the polymer backbone. PAGs comprise non-acidic molecules that form acid when decomposed through interaction with light. Acid is therefore only formed in irradiated regions of the resists. When a positive-tone photoresist is heated after such exposures, the generated acid causes catalytic cleavage of the acid sensitive side chain groups. A single acid catalyst molecule generated in this manner may be capable of cleaving multiple side chain groups, thus allowing lower exposure doses for the needed photochemical response.
Ionic PAGs have the general structure P+A−, where P+ decomposes into protons (H+) upon irradiation with light, while A− remains unchanged and forms the acid H+A−. In an efficient PAG, P+ therefore highly absorbs the photons of interest, while A− should be more or less inert to photon interaction. Because of the relatively low intensity of ArF laser source (193 nm) and relatively high binding energy of acid labile moieties in ArF photoresist, PAGs which can produce a strong Bronsted acid with high sensitivity comprise acids preferred to realize such chemical amplification in commercial lithography. The traditional way to increase acid strength of PAGs is to use sulfonic acids (R—SO3H) rather than carboxylic acids (R—COOH) and to attach strong electron-withdrawing substituents to the R group adjacent to the acid moiety. The simplest way of achieving this goal is to use perfluorosubstituted alkyl groups as R, in which fluorine acts as a strong electron acceptor. Therefore, onium salts having fluorine-containing anions A− such as perfluoroalkylsulfonate (PFAS), more specifically perfluorooctylsulfonate (PFOS) or perfluorobutanesulfonate (PFBuS), comprise photoacid generators in ArF photoresist system, in part because they result in generation of strong acid.
Examples of such commercially available PAGs comprise, for example, triphenylsulfonium nonafluorobutanesulfonate (1, “TPS PFBuS”) or bis (4-t-butyl-phenyl) iodonium nonafluorobutanesulfonate (2, “DTBPIO PFBuS”).
In recent years, there has been a desire in the microelectronics industry to eliminate the use of perfluorinated carbons (PFCs) such as PFOS and PFAS. Thus, there is a desire to find alternative photoacid generators which can be used without adversely impacting the performance of lithographic processes. In part, this desire is caused by increasing concerns about their environmental impact, such as their limited biodegradability, which makes it difficult to clear waste water from manufacturing sites using PAGs that generate perfluoroalkylsulfonic acids. The internet (http://www.itrs.net) also addresses environmental issues regarding the use of PAGs based on perfluorinated carbons. In addition, there has been a desire to minimize or eliminate fluorine content in photoresists in order to improve etch resistance and other aspects of lithographic imaging processes.
Some attempts have been made to develop photoresist formulations that do not use perfluorinated carbon-containing photoacid generators, however these have largely been unsuccessful in achieving performance comparable to formulations using PFOS. For example, TPS camphorsulfonate (3) has no fluorine in its anion A−. However, the corresponding acid, camphorsulfonic acid, is not as strong as perfluoroalkylsulfonic acids and therefore not suitable for ArF resist deprotection, i.e., cleavage of the polymer-bound acid-labile moieties, leading to changes in the resist solubility in the irradiated areas.

In general, the strength of an acid is controlled by the capability of its corresponding anion to bind a proton: the more weakly coordinating the anion, the stronger the acid. R. P. Meagley, U.S. Pat. No. 7,192,686 discloses fluorine-free PAGs based on carborane anions associated with acid-generating cations. However, the impact of therein proposed boron-containing materials, such as carborane compounds, on post-lithography steps in chip manufacturing (e.g., etching) is not known at the present time.